Concrete is generally prepared according to a mix design which specifies the proportions of the various material constituents used to produce the concrete. For example, a concrete mix commonly described as a "1:2:4" mix refers to mix proportions of one part of cement to 2 parts of fine aggregate (such as sand) to 4 parts of coarse aggregate (such as gravel). The proportion between cement and water is also usually specified. The design of the mix may depend on many factors and specific engineering requirements.
Some of these requirements relate to the rheological properties required from the mix in its fresh state during its transportation, placement and compaction. The requirements are determined to suit specific conditions of application, for example use of pumping in transportation, the measure of vibration employed during compaction, the measure of cohesion and mobility needed during placing, etc.
Whilst the mechanical properties of hardened concrete are mainly influenced by the water/cement ratio of the mix, the rheological properties of the fresh mix are strongly influenced by the water content and also strongly by the particle size distribution of the solid particles in the mix and by the relative content of fine particles, in the aggregate system.
The particle size distribution in the mix depends on the mix design as well as on the particle size distribution within each of the constituents from which the mix is produced. A change in the character or size distribution of one of the aggregate constituents of a mix having a given design, results in changes in its rheological properties.
To offset such changes, the mix design can be modified. Such modifications are commonly conducted under the supervision of a concrete technician. The suitability of the mix in its fresh state to a given application requirements is commonly measured through the shear deformation of the mix. This is commonly determined by measuring "workability" via a known test method. A common test method, known as the "Slump Test", is especially suited for the measurement of the workability of mixes having a soft consistency. A supporting frame in which a mass of concrete has been cast is lifted, allowing the concrete body to freely slump under gravity. The drop in height of the concrete mass is measured.
A test method commonly used for the definition of the "workability" of dry mixes measures the time, in seconds, it takes a body of concrete to change its shape from a truncated cone to a cylindrical shaped body under the effect of standard vibration.
Adjustments in workability are commonly achieved during production of concrete mixes by changing the quantity of water in the mix.
Also common in the monitoring of workability of a concrete mix whilst in a mixing drum is the measurement of the force required to mix the concrete by rotation of the drum or the paddles in the drum. A measurement of the force can be obtained by monitoring the hydraulic pressure or current needed to operate the mixer motor.
The resistance to deformation of a mix can be regulated by changing the amount of water added to it. It is known how to effect an automatic adjustment to the amount of water in the mix so that a constant workability is maintained. For example, European Patent Publication 0 126 573 A1 to Durant proposed a method of controlling the quality of the concrete mix in mobile mixers by measuring the workability of the concrete mix and selectively adding water to achieve the required consistency for the concrete mix.
Different concrete mixes can exhibit equal workabilities when measured by different techniques and yet can possess totally different rheological properties relating to their suitability for commonly required applications. For example, when a concrete mix is designed, for pumping, its rheology is especially adjusted to be pumpable under the set of conditions.
A standard mix design "assumes" that the characteristics of the various constituents are constant. However, in practice, the characteristics of a specific aggregate may vary in time. It may become necessary to change the mix proportions in order to maintain the desired mix characteristics. Unfortunately, present technology does not have any objective procedure of accurately defining the rheological requirements and methodically making changes to the mix proportions so as to ensure that the final product has the required characteristics. The changes, when made, are according to a subjective and qualitative assessment of a concrete technician. It is known to alter the amount of water within a concrete mix in order to change the workability of the mix.